Such modules are known in a wide variety of variants and mostly comprise a housing with one or more filter elements positioned therein which divide the space inside the housing in a feeding side and permeate side. One type of filtration modules is based upon so-called dead-end filtration in which fluid to be filtered is fed to the feeding side, from there is passed through the filter element(s), the solids being trapped in the filter and permeate (filtered fluid) is released at the permeate side. Another type is based upon so-called cross-flow filtration in which the majority of the fluid flows tangentially across the filtering surface rather than into the filter. The fluid is passed at positive pressure relative to the permeate side. A portion of the fluid which is smaller than the pore size of the filter passes there through as permeate, everything else is retained on the feeding side as retentate. The tangential motion of the bulk of the fluid across the filter causes trapped particles to be rubbed of the filtering surface. This means that a cross-flow filtration module can operate for a relative long period of time at relatively high solids loads in the fluid without the filter getting blinded.
EP-0 208 450 shows an embodiment of a cross-flow filtration module that is used for the filtration of beer. This module comprises a longitudinal cylindrical housing inside which a bundle of tubular ceramic membrane elements is provided. Unfiltered beer is fed via a fluid feed at the lower side of the module and there enters the inner feeding spaces of the tubular membrane elements. Clear permeate passes through the membrane walls, there enters a permeate space which is left clear around the outer sides of the membrane elements, and from there is drawn of via permeate outlets. Retentate is drawn off via a retentate outlet at the upper side of the module where it leaves the inner feeding spaces of the tubular membrane elements again. The retentate is then circulated via a cooler and a pump back again to the fluid feed at the lower side of the module.
It is a known phenomenon with cross-flow membrane filtration modules that there is a pressure drop in the fluid flow between the fluid feed and the retentate outlet. This pressure drop has the effect that the flux of permeate flowing through the membrane walls also differs over the length of the membrane elements. Close to the fluid feed the fluid pressure at the feeding side is higher and consequently the flux also, whereas closer to the retentate outlet the fluid pressure at the feeding side is lower and consequently the flux also. If the average pressure difference over the membrane walls between the feeding side and the permeate side, the so-called Trans Membrane Pressure (TMP), is relative low, it can even occur that over a part of the length of the membrane elements closest to the retentate outlet, a negative flux starts occurring because there the local pressure of the permeate at the permeate side has gotten higher than the local pressure of the fluid at the feeding side. The local TMP there has gotten negative. In other words, over this part of the length of the membrane elements, already filtered permeate starts flowing back from the permeate side towards the feeding side. This of course is very undesirable because it has a negative impact on the performances of the entire module. It decreases the net permeate yield or, if the module is controlled on the basis of a constant net yield in permeate, it increases the local flux near the fluid feed. This last phenomenon is caused by the fact that the negative local flux near the retentate outlet needs to be compensated for by that part of the membrane elements which still has a positive flux. All in all this can even have the effect that the minimum and maximum local fluxes come to lie so far apart that the maximum flux gets more than 100 times higher than the average flux. The higher local flux near the fluid feed has the effect that this part of the membrane elements gets contaminated far more quickly, which in turn substantially reduces the time that the module can be used for filtration before it needs to be cleaned.
US 2007/0158256 shows a filtration module having a membrane with on the left a feeding side with a fluid feed and a retentate outlet, and with on the right a permeate side with a permeate outlet. During filtration a fluid to be filtered is fed via the fluid feed into the feed side. This fluid partly permeates through the membrane towards the permeate side and there is removed as permeate via the permeate outlet. The part of the fluid which does not permeate through the membrane is removed as retentate via the retentate outlet. Both at the feed side and at the permeate side a cross-flow of fluid flowing along the membrane is achieved during filtration. On the feed side this is obtained by means of the provision of a retentate circulation line with a pump provided therein. On the permeate side this is obtained by means of the provision of a permeate circulation line with a pump provided therein. The forced flow in both circulation circuits is such that the pressure drop along the membrane is substantially equal over the whole surface of the membrane. During a filtration process the membrane is prone to get fouled. Therefore, from time to time, the membrane needs to get cleaned. This cleaning is done by closing a valve which is provided inside the permeate outlet at a high frequency. Each time the valve closes a pressure quickly builds up at the permeate side which is higher compared to the pressure at the feed side. This results in a temporary reversal of the fluid flow inside the membrane. Owing to the high frequency with which the valve is closed, a so-called high frequency back-pulsing of permeate thus takes place inside the membrane which high frequency back-pulsing each time briefly interrupts the filtration process. By combining this high back-pulse frequency with the cross-flow on both sides of the membrane during filtration, the membrane can be kept relative clean. The substantially constant pressure drop along the membrane during filtration helps to avoid a backflow of permeate during the filtration process.
A disadvantage, however, is that the extra pumps needed to obtain the required cross-flow/sweep flow along both sides of the membrane, make the installation expensive, consume lots of energy, and thus makes this filtration module rather expensive during use.
US 2009/0069619 schematically shows a membrane module having a fluid feed, a retentate outlet and a permeate outlet. A part of the fluid which is delivered via the fluid feed by-passes the membrane module unfiltered via a branch and is mixed with filtered permeate coming out of the permeate outlet. This mixture of filtered permeate and unfiltered fluid then arrives in an operation unit, such as an isomerisation reactor, after which it leaves the system as a treated product stream. The by-passing of unfiltered fluid aims to provide a high purity retentate fraction with less membrane surface area than would be required without the by-pass.
Besides the disadvantage that only non-critical permeate mixtures can be obtained, another disadvantage is that the membrane module needs to be thoroughly cleaned periodically. Further it is noted that in this known method also it can occur that over a part of a filter element inside the membrane module, a negative flux starts arising because there the local pressure of the permeate at the permeate side has gotten higher than the local pressure of the liquid at the feeding side. This is very disadvantageous because it has a direct negative impact on the aimed high-purity retentate fraction which is diluted again by the back flowing permeate.
US-2009/0217777 shows a method of concentrating an analyte that is present in a liquid. This method starts with the concentration of the liquid by passing it through a filtration module of which one or more ultra filter membranes are such that the analyte is unable to pass through it. Filtered liquid component not containing the analyte then forms permeate, while the liquid and analyte staying behind form retentate. After a certain period of time the filtration process is ended, after which the collecting of the thus formed retentate is started. As a first step in this collection the permeate side of the module is flushed with a gas. Subsequently the retentate side is flushed with a mixture of a gas and a liquid. The retentate solution flushed out can then be collected and includes the concentrated analyte.
It is noted however that in this known method also it can occur that over a part of the length of the ultra filter membranes, a negative flux starts because there the local pressure of the permeate at the permeate side has gotten higher than the local pressure of the liquid at the feeding side. This is very disadvantageous because it has a direct negative impact on the aimed concentration of the analyte which is diluted again by the back flowing liquid.